When component parts are held in a jig they frequently must be precisely positioned. Some jigs have scales along their peripheral sides to which clamps are mounted. In many cases it simplifies positioning if one need only position the clamps and then are thereby able to position all work pieces which are later held by the clamps.
On other occasions it is desirable to rotate the clamp holding the work piece in order to rotate the work piece. With most clamps it is impossible to rotate the work piece without changing its position. Even when a symmetrical work piece is rotated, it is usually necessary to reposition the clamps holding it in the jig.